The present invention relates to a method of manufacturing a plano lens particularly suitable for use in a screen of a back projection type projector.
In recent years, as a large screen display used for a high definition television or a theater, a back projection type projector using a liquid crystal light bulb or a CRT has been actively developed.
FIG. 30 shows a schematic configuration of a related art back projection type projector.
Referring to FIG. 30, there is shown a box type projector in which image light L projected from an image projecting unit 101 is reflected by a reflecting mirror 102 to be introduced to a transmission screen 105. The transmission screen 105 is composed of a Fresnel lens 103 which is generally combined with a vertically extending lenticular lens 104. Image light L incident on the backface of the transmission screen 105 is substantially collimated through the Fresnel lens 103, and the parallel rays of image light L are diffused mainly in the horizontal direction through the lenticular lens 104.
As shown in FIGS. 31A and 31B, the lenticular lens 104 has on its back side (light outgoing side) vertically extending projections 104a on which are provided black stripes 104b for absorbing external light thereby improving the screen contrast. For example, an acrylic resin is extrusion-molded into the shape of the lenticular lens 104 including the projections 104a, followed by printing of only the projections 104a with a black color, to form the black stripes 104b.
As shown in FIG. 31B, a width "w" of each black stripe 104b is, generally, set at a value being 0.3 to 0.4 time an arrangement pitch "p" of the projections 104a of the lenticular lens 104.
The transmission screen using the above lenticular lens has a disadvantage that it allows diffusion of rays of light in a wide range in the horizontal direction, leading to a wide viewing angle in the horizontal direction; however, it allows diffusion of rays of light only in a narrow range in the vertical direction, leading to a narrow viewing angle in the vertical direction. To solve such a disadvantage, there has been known a structure having a vertically extending lenticular lens combined with a horizontally extending lenticular lens. Such a structure, however, has problems that the increased number of the parts raises the parts cost and manufacturing cost, the increased number of the stacked lenticular lens layers of the screen thickens the screen, and the influence of multiple reflection between the lenticular lens layers becomes larger.
The transmission screen using the lenticular lens has another disadvantage. To provide the black stripes for improving contrast as described above, the projections for printing a black color must be formed on the light outgoing side of the lenticular lens, and in this case, the width of each projection must be set at such a value as not to interfere with rays of outgoing light. Consequently the area ratio of an external light absorbing unit composed of the black stripes is generally set at about 30 to 40%, and thereby the effect of improving contrast is relatively poor.
To cope with the above disadvantage, attention has been given to a transmission screen using a plano lens with minute transparent balls two-dimensionally arranged in place of the above lenticular lens (for example, disclosed in U.S. Pat. Nos. 2,378,252 and 3,552,822, and Japanese Utility Model Registration No. 2513508). Such a transmission screen using a plano lens has been studied to be practically used for a high definition large screen display.
A configuration of the above transmission screen using a plano lens, having been proposed by the present applicant in Japanese Patent Laid-open No. Hei 9-100590 (filed on Apr. 17, 1997), will be described with reference to FIGS. 32 to 34.
FIG. 32 shows a main configuration of a back projection type projector of an open type in which rays of image light L projected from an image projecting unit 21 are diffused forward via a transmission screen 10 composed of a Fresnel lens 22 and a plano lens 23. As shown in the figure, the plano lens 23 is configured such that minute transparent balls 2 are two-dimensionally disposed in a closet packing array. Accordingly, only one layer of the minute transparent balls 2 allows rays of the image light L to be diffused in a wide range in each of the horizontal and vertical directions.
FIG. 33 shows a back projection type projector of a box type in which image light L projected from an image projecting unit 21 disposed in a housing 25 is reflected by a reflecting mirror 24 and is diffused forward via a transmission screen 10 having a Fresnel lens 22 and a plano lens 23 composed of minute transparent balls 2.
FIG. 34 shows one example of a configuration of the above plano lens 23.
The plano lens 23 is configured such that a number of minute transparent balls 2 such as glass beads are held in a transparent adhesive layer 5 formed on a light incoming side transparent substrate 4 with about 50% of the diameter of each minute transparent ball 2 buried in the transparent adhesive layer 5. A light absorbing layer (coloring layer) 3 made from carbon toner is formed such that gaps between respective adjacent ones of the minute transparent balls 2 are filled with the light absorbing layer 3 and the top and its neighborhood of each minute transparent ball 2 on the light outgoing side are exposed from the light absorbing layer 3. On the minute transparent balls 2 on the light outgoing side is stacked a transparent substrate 1 via a transparent adhesive layer in order to protect the minute transparent balls 2 and the light absorbing layer 3 from the external environment.
Rays of incoming light L.sub.in, which have been made incident on the plano lens 23 via the Fresnel lens (not shown), pass through the transparent substrate 4 and the transparent adhesive layer 5 on the light incoming side and are converged through each minute transparent ball 2, as shown in FIG. 34; and the converged rays of light pass through the top and its neighborhood of each minute transparent ball 2 on the light outgoing side and also pass through the transparent adhesive layer 6 and the transparent substrate 1 on the light outgoing side, and go out of the plano lens 23 as diffused rays of outgoing light L.sub.out. On the other hand, rays of external light L.sub.ex, having been made incident on the plano lens 23 from the transparent substrate 1 side are almost absorbed by the light absorbing layer 3, to thereby reduce lowering of contrast due to reflectance of the rays of external light L.sub.ex.
At this time, in the plano lens 23, the area ratio of the light absorbing layer 3 on the light outgoing side can be set at about 80% or more, so that lowering of the contrast due to reflectance of the external light L.sub.ex can be significantly reduced. Thus a screen having a high contrast, being less affected by external light, can be realized.
The plano lens 23 is manufactured, for example, as follows.
The transparent adhesive layer 5 is formed on the transparent substrate 4 on the light incoming side and a number of the minute transparent balls are scattered on the transparent adhesive layer 5. The minute transparent balls 2 are pressed down such that about a half of the diameter of each minute transparent ball 2 is buried in the transparent adhesive layer 5. A powdery light absorbing material such as carbon toner is scattered over the surface of the minutes transparent balls and is pressed thereon by, for example, a pressing roll to fill gaps between respective adjacent ones of the minute transparent balls 2, to form a light absorbing layer 3. Then, part of the light absorbing material present on the tops and their neighborhoods of the minute transparent balls 2 on the light outgoing side is wiped off, to form light outgoing portions of the minute transparent balls 2. After that, a transparent substrate 1 is stacked on the light outgoing side via a transparent adhesive layer 6.
The above manufacturing method, however, has a disadvantage. Since there may occur slight inevitable variations in diameter and buried depth of the minute transparent balls 2, it is difficult to accurately remove part of the light absorbing material on the light outgoing portions of the minute transparent balls 2. As a result, the transmission screen 10 using the related art plano lens 23 is relatively large in brightness and unevenness of contrast.
Another problem of the related art method is to require a lot of labor and working time for wiping off the light absorbing material.